(1) Field of the Invention
The present invention relates to a sliding member, especially, to the wear resistant sliding member processed with a surface treatment.
(2) Description of the Prior Art
In order to enhance wear resistivity of sliding members of machines, improvement of the sliding members themselves and surface reforming of sliding portions etc. are performed.
As for the surface reforming of sliding portions, methods for forming a hard coating on the surface of the sliding members by such methods as hard chromium plating, ceramic coatings, and hard anodic oxidation coatings etc. are well known. Recently, forming metallic coatings having a low melting point or organic polymer coatings on the sliding surface are adopted in order to give a lubricating property in addition to wear resistant property.
When hard coating is formed on the surface of the sliding members, wettability with lubricating oil or the holding property of the lubricating oil become problems. As for the reforming coating having a preferable holding property of the lubricating oil, the hard anodic oxidation coatings can be exemplified. Alumite layer formed by anodic oxidation on an aluminum alloy has a great number of fine vertical holes, and has a preferable holding property of lubricating oil because of impregnation of the lubricating oil into the vertical holes. However, if the diameter of the vertical hole is too small and viscosity of the lubricating oil is too high, the holding property of the lubricating oil is decreased. Accordingly, it is important to control the diameter of the fine vertical hole so as to obtain a proper size of the holes matched to the viscosity of the lubricating oil.
In order to make the wear resistant property compatible with the lubricating property, there is a method for forming a surface layer with organic material or soft metal having a preferable lubricating property, wherein hard particles are dispersed. However, the above described material has such a problem that the hard particles are separated from the matrix layer in accordance with the proceeding of abrasion and cause abrasive wear by entering into frictional planes. Accordingly, it is important to prevent separation of the hard particles from the matrix layer.
As for a rotary drum for videotape recorder composed of sliding members made from an aluminum-silicon alloy, a sliding member, which is improved in sliding characteristics by forming and projecting fine silicon crystalline particles having a particle size smaller than a submicron on a surface layer of the sliding portion of the drum whereon the tape slides, is proposed (JP-A-57-147155 (1982)). In this case, the hard silicon particles which are dispersedly formed at the surface of the aluminum sliding material enhance a gas (air) lubricating effect by projecting out from the surface of the aluminum matrix, but, on the other hand, there is such a problem that failure of the friction planes is easily caused by separation of the hard silicon particles.
One of the methods to solve such a problem is, for example, a method to form a porous surface structure by vacuum vapour deposition of zirconia at an elevated temperature higher than 500.degree. C. However, almost all of the many pores formed by the above described method are independent pores and are not connected with each other. Additionally, as a process to treat the sliding member at high temperature is indispensable for forming the porous surface in the above described method, the method is not applicable to such members as a member which has previously been processed by heat treatment (quenching or annealing), a member which will cause distortion by heating or cooling, or a member composed from metal having a low melting point.